Giyu Tomioka, the master who would one day train Zenitsu in the Thunder Breath technique, had already begun to make his mark on the world. His crackling electricity could split the air itself, leaving scorched earth in the wake of his battles.
Meanwhile, Urokodaki Sakonji continued his quiet but vital work in the mountains, training promising young warriors in the Water Breathing forms. Among his students were future legends-Tomioka Giyu, whose calm demeanor masked incredible skill; Sabito, whose fierce determination drove him to push himself beyond normal limits; and Makomo, whose graceful movements made water seem like liquid poetry.
Even Kamado Tanjuro had been born into this changing world, carrying the bloodline that would one day produce the most important demon slayer in history.
On Muzan's side, desperate circumstances had forced him to create new Lower Moons to replace the constant losses he suffered. Names like Rokuro and Kamanue appeared briefly in intelligence reports before being quickly eliminated. The Demon King's recruitment efforts had become a futile exercise - each new demon he created was almost immediately hunted down by his growing list of enemies.
It was a bitter irony. Muzan had too many enemies and not enough time to properly train his subordinates before they were destroyed.
The chaotic situation had reached such extremes that the Taisho government officially published a comprehensive guide documenting all known factions and their current activities. Citizens could purchase monthly updates and study the latest movements of demons, heirs, and demon hunters as if they were following a complex military campaign.
For the demon world, all secrecy was gone. Every shadow had been dragged into the harsh light of day.
Everyone knew that Muzan Kibutsuji was the progenitor of all demons, the source from which centuries of horror had flowed. His name was spoken with the same mixture of fear and hatred that people reserved for natural disasters.
The Heirs, meanwhile, were branded as cultists who followed the twisted philosophy of the "God of Swordsmen. They preached that the current world was fundamentally broken and in need of complete reconstruction through demonic transformation. According to their doctrine, only by embracing the power of demons could humanity transcend its pitiful limitations and achieve true evolution.
The God of Swordsmen they referred to was, of course, Oboro himself - though the general population refused to believe this connection.
In human culture, the figure known as the God of Swordsmen represented something quite different. He was a deity of righteous power, a bringer of hope who stood firmly on the side of humanity. The warm, encouraging presence that had appeared throughout their history bore no resemblance to the cold manipulator the Heirs served.
The cognitive dissonance was so complete that most people assumed the Inheritors were simply delusional fanatics who had stolen the name of a benevolent God to legitimize their twisted crusade.
As for the Demon Slayer Corps, their centuries of secrecy had finally ended. No longer did they operate out of hidden compounds and secret training grounds. Government support had transformed them from an underground resistance into an official demon-hunting organization, complete with public recruitment drives and standardized training programs.
To accommodate this new reality, fortified settlements had been built throughout the country. High walls and organized patrols concentrated scattered populations into defensible positions, making it much harder for demons to pick off isolated victims. The days of living as lone mountain hunters or remote farmers were quickly coming to an end.
The small town where Oboro had made his home was a perfect example of these changes. Despite its modest size, massive stone walls surrounded the entire community, with watchtowers and sentry posts placed at regular intervals. Armed patrols walked the walls at all hours, their movements coordinated by a system of bells and signal mirrors that could summon reinforcements within minutes.
The change was remarkable. While the Demon Slayer world retained its basic character and historical atmosphere, every aspect of society had been revolutionized. The power structures, social organization, and underlying systems of power had evolved into something completely unprecedented.
The metaphorical tree representing the will of this world had finally begun to bear fruit-small, green, and still evolving, but unmistakably real.
Still, the harvest remained incomplete. Oboro could sense that his time here was coming to an end, whether the traditional plot ever began or not. The seeds he had planted over decades of careful cultivation were nearing maturity, and some were ready for harvest.
Some of his experiments had succeeded beyond his wildest expectations. Others had been disappointing failures. It was time to evaluate what could be salvaged and what had to be abandoned.
First: The Power of the Soul
This seed had yielded mixed results. While Oboro had successfully elevated the entire demonic species to a new evolutionary level through Soul Force techniques, he had failed to create anything truly innovative. Every application of Soul Force had been derivative, following patterns he had already established rather than breaking new ground.
From a pure research perspective, this was a significant failure. He had hoped that the widespread distribution of Soul Force would lead to unexpected discoveries, new training methods, or revolutionary applications that even he hadn't considered. Instead, the demons had simply copied his existing techniques without adding any meaningful improvements.
Therefore, this particular line of development would have to be recycled. Regardless of whether Muzan survived the coming conflicts, all of his blood demon arts, fighting techniques, and spiritual blood would be transformed into experience cards for transport to the next world. The knowledge wouldn't be lost, but it wouldn't remain in this reality either.
However, from the perspective of improving the overall power system of the Demon Slayer's world, the soul power experiment had been extremely successful. The fundamental changes it introduced would continue to influence the development of this reality long after Oboro's departure.
Second: Gyutaro
Like Muzan, Gyutaro's contributions to the overall growth of the world were undeniable. His endless battles had advanced combat techniques through practical application and deadly innovation. But his failure to transcend the limitations of normal martial arts meant that pure physical combat would always remain confined within the boundaries of this world.
Despite intense study by multiple factions, despite countless observers analyzing his movements and attack patterns, no one had managed to evolve beyond mere imitation. They could learn his footwork, copy his blade techniques, even develop variations of his fighting style-but none had achieved the breakthrough that would elevate martial arts to a truly transcendent level.
The fundamental problem was that the Demon Slayer world lacked a sufficient martial arts foundation to support such an evolution. Aside from the late Akaza, there were virtually no examples of superior physical technique from which to draw inspiration. Even the ninja traditions that existed in this reality were crude compared to the sophisticated assassination arts of families like the Zoldycks.
Gyutaro's presence had enriched the world's martial foundations, but that influence would always remain a niche specialty rather than a revolutionary force.
Third: Breathing Techniques
This seed had brought about the most spectacular success.
Tanjiro Kamado, now in his sixties, had achieved something Oboro hadn't even thought possible - the cultivation of "qi points" throughout his body. The development was similar to the legendary "opening of the Ren and Du meridians" of ancient Earth traditions, but with crucial differences that made it uniquely suited to the conditions of this world.
Through decades of patient experimentation, Tanjiro had learned to focus his breath energy on specific acupuncture points, storing concentrated power that could be released in explosive bursts. The stored qi could be maintained indefinitely and released at will, granting inhuman strength and speed when circumstances demanded it.
Even more remarkably, this breakthrough had been achieved entirely through independent research. Oboro had provided no guidance, offered no hints - Tanjiro had unlocked these secrets through sheer determination and inherited wisdom.
Meanwhile, the Demon Slayer Corps had developed their own parallel advancement called the Tonkotsu Vortex. Instead of storing energy in specific points, this technique distributed concentrated qi throughout the muscles and bloodstream, creating microcirculatory systems that functioned like biological engines. As long as the practitioner maintained proper breathing patterns, every part of the body could function at an enhanced capacity.
The philosophical difference was intriguing. Tanjiro's qi points emphasized explosive release-gathering energy over time and unleashing it in devastating bursts. The Tonkotsu vortexes of the Corps focused on sustained improvement-continuous improvement of all physical abilities.
Both approaches had merit, and both had been developed entirely within the natural limitations of this world. As Oboro studied these techniques through his system's analytical capabilities, the results exceeded all expectations.
The system had spent a long time processing the data, comparing patterns with his memories of Earth's martial traditions. When the calculations were finally complete, the results had truly stunned him.
An improved version of breathing techniques that went beyond the scope of any national martial art he'd ever encountered. Something that approached the theoretical ideal he'd dreamed of but never dared to hope for.
Core Tenkai-the cultivation of a sea of energy in the lower abdomen, related to but distinct from ordinary breathing techniques. A method that bore unmistakable similarities to Earth's most esoteric traditions, whether those traditions had actually existed or were merely products of his imagination.
The development represented a fundamental transformation: from simple breathing exercises to true qi refinement. From physical improvement to spiritual cultivation.
Oboro had been pleased with the development, though not to the point of ecstasy. Any technique that included the word "method" in its classification indicated a power that surpassed most higher world systems. But it couldn't guarantee absolute safety in unknown dimensions, where hostile forces might not allow time for proper cultivation.
The irony wasn't lost on him. In the supposedly low-level Demon Slayer world, he had successfully tapped into upstream power sources that had eluded him in the Hunter x Hunter reality. What should have been impossible had become inevitable through the pure power of inherited will.
Both the Kamado Clan and the Demon Slayer Corps shared the concept of "inheritance"-knowledge passed down through generations, refined and improved by each practitioner who added his or her own understanding to the collective wisdom. This continuity, this dedication to preserving and advancing techniques over decades or centuries, had created something greater than the sum of its parts.
If the members of the Demon Slayer Corps hadn't been driven by their mission to eliminate demons, they would never have devoted so much effort to breathing research. If the Kamado clan hadn't been obsessed with perfecting sun breathing techniques, Tanjiro would never have made his current breakthroughs.
Necessity had bred innovation. Desperation had bred transcendence.
"If someone could truly master this method and achieve true Core Tenkai cultivation," Oboro mused to himself, "they could become a 'land immortal' even in higher worlds.
The estimate was not made lightly. In all of his previous adventures, he had only been able to sense and manipulate qi as an external force. The concept of developing an internal sea of energy, of transforming one's body into a self-contained cultivation system, had always seemed impossibly metaphysical.
But now the system had provided detailed training protocols, step-by-step instructions for achieving what had once been pure fantasy.
"What I couldn't achieve in Hunter x Hunter has been realized in Demon Slayer," he whispered in amazement. "It's almost unbelievable..."
But underneath his excitement was a persistent stream of concern. "The problem isn't just the energy systems and environmental factors of the next world. It's about the will of that world itself. If it decides to eliminate a dimensional stowaway like me before I can establish myself... even with a path to land immortal status, I might die before I take the first step."
Oboro understood his precarious position better than anyone. The will of the Demon Slayer world had been cooperative, even helpful, because it lacked the power to simply vanquish him. Their relationship had been based on mutual benefit rather than power dynamics.
But in other realities, especially higher worlds with more sophisticated dimensional defenses, he could be detected and classified as a virus the moment he arrived. His very presence could be interpreted as a threat to universal stability, an unplanned variable that required immediate elimination.
To the governing will of any world, a traveler like him represented potential chaos. His actions could shift entire timelines, derail predetermined destinies, or introduce power systems that weren't meant to exist in that reality. From their perspective, the safest response would always be immediate destruction.
Therefore, despite his recent breakthrough, he couldn't allow himself to become overconfident. The method of refining qi was invaluable, but it would be useless if he died before mastering even the basic stages.
The key would always be his soul and the system's abilities. These were the only tools he could rely on in the critical first moments of any new world. Only after he had established a secure foundation with these two powers could he begin to develop other techniques and abilities.
Everything else-no matter how impressive or revolutionary-would always be secondary to the fundamental challenge of survival.